Abstract: The carbohydrate antigen CA50 was examined in serum level of 45 normal persons, 201 pa-tients with benign diseases and 336 patients with malignannt tumor by immumoradiometric assay (IRMA). The results showed that the mean CA50 value in the patients with liver cancer, colon can-cer and lung cancer was much higher then that in the nomal control group (P<0.01). Taking 20ku/L as the upper limit of nomal range, the positive rate were 79.1%(53/67) of patients with liver cancer. 36.4%(16/44) in colon cancer, 34.9%(29/83) in lung cancer. From these results, we concluded that the measurement of level of serum CA50 may be an useful marker for malignant tu-mors diagnosis.